The concept of converting heat to sound has been known for over two hundred years. For example, in the “singing pipe,” heat is applied to a closed end of a resonant tube having a metal mesh within the tube which has a “hot” end near the heated end of the resonant tube and a “cold” end further from the heat source. The terms “hot” and “cold” refer to their relative temperatures with respect to each other. The “hot” end could be at room temperature with the important parameter not being the actual temperature, but the temperature gradient.
An acoustical standing wave set up in the resonator tube forces a working fluid (e.g., a gas) within the resonator to undergo a cycle of compression, heating, expansion, and cooling. In this case, thermal energy is converted into acoustical energy and it maintains the standing waves.
The work of converting heat to sound has been moved forward through the development of thermoacoustical refrigerators, as disclosed in U.S. Pat. No. 6,574,968, entitled HIGH FREQUENCY THERMOACOUSTIC REFRIGERATOR, which is incorporated herein by reference. Essentially, the conversion of heat to electricity by the present invention can be thought of as the opposite process performed by the thermoacoustic refrigerator. Thus, instead of applying energy to a piezoelectric element to thereby cool a device, energy is being taken and converted from a heat source itself.
Early attempts to create a thermoacoustic energy converter have failed for various reasons. For example, the process was performed in prior art devices operating at around 100 Hz which would convert the low frequency sound to electricity. However, the process was abandoned by those skilled in the art because of the very low efficiency of the energy conversion process at low frequencies.
One prior art process for direct conversion of heat to electricity utilizes a permanent magnet and a moving coil. This process is costly because of the magnet. It is also bulky and heavy and the efficiency decreases as the frequency of the device increases, making high frequency operation impractical. The device itself can also cause magnetic interference with nearby magnetically sensitive devices, precluding use in certain environments.
In order to make a thermoacoustic energy conversion process practical, it may be desirable to operate the device at high frequencies. High frequencies can result in more efficient operation of an electro-mechanical transducer, such as a piezoelectric element that is to be used in the present invention for the conversion of sound energy to electricity.
Another advantage of operation at high frequencies comes from a comparison with prior art thermoacoustic devices that are relatively large compared to semiconductor devices and biological samples. Thus, it would be another advantage to make the thermoacoustic energy converter small enough to be operable with such devices and samples.
Attempts to address the shortcomings of the prior art have resulted in devices, such as that disclosed in the published International Patent Application entitled High Frequency Thermoacoustic Energy Converter, International Publication Number WO 03/049491, which is incorporated by reference herein in its entirety. Such devices addressed the problems with other prior art devices by using a resonator that also functions as a housing for an electro-mechanical transducer, a regenerator formed from random fibers comprised of a material having poor thermal conductivity and a pair of heat exchangers comprised of a material having good thermal conductivity positioned on opposite sides of the regenerator. The energy converter utilizes a standing wave within each resonator, which limits the efficiency of the device.
Another attempt is disclosed in the published U.S. patent application entitled Compact Thermoacoustic Array Energy Converter, U.S. Publication. Number 20090184604A1, which is incorporate by reference herein in its entirety. In this application, a plurality of heat driven thermoacoustic prime movers are arranged in parallel, coupled by means of an acoustic cavity to a piezoelectric electrical generator whose output is rectified and fed to an energy storage element. The prime movers convert heat to sound in a resonator. The sound form a phase-locked array is converted to electricity by means of the piezoelectric element. The generated electric energy is converted to DC by means of a rectifier set and it is then stored in a battery or supercapacitor. Again, this type of device generates a standing wave within the resonator, which limits efficiency.
In 1979, P. Ceperley introduced the concept of a traveling wave thermoacoustic device for improved efficiency. This is in contrast to the standing wave device where a phase shift is needed to provide power output. It is achieved by heat flow between a regenerator of high surface area elements and the generated sound field, which introduces irreversibilities. In a traveling wave device there is no need for a phase shift between acoustic pressure and acoustic speed, and hence a high efficiency is expected. Ceperley found that acoustical energy could be generated by differentially heating a regenerator within a wave guide. Because of such heating, the gas in the regenerator region undergoes a Stirling thermodynamic cycle similar to a Stirling cycle where air acts like a piston. Ceperley also found that if the acoustic wave propagates from cold to hot inside the regenerator acoustic amplification occurs; i.e. the heat gradient is driving the wave and it amplifies the acoustic power. Such a device is a thermoacoustic prime mover. The regenerator replaces the stack in the standing wave device; its function is to exchange heat with the sound wave, isothermally, i.e. with ωτ<<π where ω is the angular frequency of the sound wave and τ is the thermal relaxation time between regenerator and sound field. Ceperley also calculated the ideal gain and energy conversion for these devices. This work stimulated much research, experimentally and theoretically, as well as numerical investigations.
Prior developments have shown the performance of a looped tube traveling wave engine. It had an average length of 2.58 m and the frequency of the gas oscillations was 268 Hz3. Further development of this type of engine by the Los Alamos group led to a device which included a resonator. Their device was 5.00 m long and it operated with helium gas at 30 atm. Its resonant frequency was 80 Hz. The experimental results demonstrated up to 50% more efficiency than its standing wave counterpart, reaching efficiencies of 41% of Carnot. A limiting factor in efficiency was the problem of streaming which was subsequently suppressed. Due to the low acoustic impedance of the working gas viscous losses were high. This was solved by increasing the gas impedance by adding a standing wave component.
Electronic devices and machinery produce waste heat which limits their performance and efficiency. Thermal management of such heat and its conversion to electrical power would raise their output and at the same time provide an important source of renewable energy. Achieving such goals with simple, efficient and high power density devices would assist in providing a solution to current energy problems. The effectiveness of such an approach will be determined by the nature of the devices, on their ability to cope with a wide range of heat inputs from waste heat, and on their impact on the environment.
Problems that need to be solved deal with device interfacing to the source of waste heat and device scaling to a wide range of heat sources including compact electronics. Moreover with escalating power levels in waste heat, it is important for the devices to be high power density units in order to cope with high power level demands. Thus, there is an ever-increasing need for more energy to be reduced by providing renewable energy from waste heat. As there is an abundance of such waste heat, an efficient technology is needed for converting the waste heat to electricity. Such a technology would be capable of interfacing with sources of waste heat, would have an extended life and would be relatively inexpensive to manufacture and implement. A system or method capable of addressing these issues and of handling the dual function of energy conversion and thermal management for a wide range of applications would be an improvement in the art.